Prepare changelog for 0.12.0

changelog/0.12.0_2021-02-14/issue-2688

@@ -0,0 +1,31 @@
+Bugfix: Make `backup` and `tag` commands separate tags by comma
+
+Running `restic backup --tag foo,bar` previously created snapshots with one
+single tag containing a comma (`foo,bar`) instead of two tags (`foo`, `bar`).
+
+Similarly, the `tag` command's `--set`, `--add` and `--remove` options would
+treat `foo,bar` as one tag instead of two tags. This was inconsistent with
+other commands and often unexpected when one intended `foo,bar` to mean two
+tags.
+
+To be consistent in all commands, restic now interprets `foo,bar` to mean two
+separate tags (`foo` and `bar`) instead of one tag (`foo,bar`) everywhere,
+including in the `backup` and `tag` commands.
+
+NOTE: This change might result in unexpected behavior in cases where you use
+the `forget` command and filter on tags like `foo,bar`. Snapshots previously
+backed up with `--tag foo,bar` will still not match that filter, but snapshots
+saved from now on will match that filter.
+
+To replace `foo,bar` tags with `foo` and `bar` tags in old snapshots, you can
+first generate a list of the relevant snapshots using a command like:
+
+    restic snapshots --json --quiet | jq '.[] | select(contains({tags: ["foo,bar"]})) | .id'
+
+and then use `restic tag --set foo --set bar snapshotID [...]` to set the new
+tags. Please adjust the commands to include real tag names and any additional
+tags, as well as the list of snapshots to process.
+
+https://github.com/restic/restic/issues/2688
+https://github.com/restic/restic/pull/2690
+https://github.com/restic/restic/pull/3197